The Sparkfun CAN Shield only does HSCAN. If you want to use it for GMLAN you will have to use CAN H and put that to pin 1 on the OBDII (SWCAN) and then tie CAN L to ground, then set the appropriate baud rate (33.3K). This is the "quick and dirty" way of getting SWCAN to work with the high speed MCP2551 controller. Technically it should have a TH8056 or similar for dedicated and compliant SWCAN/GMLAN (plus it adds the ability to do a 0x100 HV wakeup)

Ahh alright cool. So yeah, the 'quick hack' is to geth the can shield to work on gmlan. So if we design our own board, will would want to lie canL to ground and canH to gmlan pin.

Too easy. But now if we want to harness both gmlan and standard CAN (High 500, Low 250).. then I would need a how other set of mcp2551 and mcp2515.. since they will be dedicated and working on separate networks.

Hmm.. its looking to be a bit more dificult to fit them onto one board then what I originally thought.. could try double sided..or just two separate boards that are stackable.?

This is currently what Im looking at, based off the sparkfun design and parts:

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You will need a MCP2551 for HSCAN as its a dual wire HS controller and a TH8056 for GMLAN because its a low speed dedicated GMLAN/SWCAN controller. Using the MCP2551 as a SWCAN controller is going to work but its a hack and you are slowing it right down to get it to run at 33.3k, plus tying the CAN L to ground. a TH8056 comes out as one wire, hence Single Wire CAN

You could swap out the MCP2551 for the TH8056 as they are almost pin compatible BUT you will have to set the Mode0 and Mode1 pins so it operates in low speed 33.333k. High speed 83.33k i have never seen actually used on any module yet it is in the specs so don't bother

Sooo the TH8056 is the CAN transceiver.. just like the mcp2551. But, from what I can tell, there is no CAN controller by that company that directly interfaces with it.

Im assuming.. that I should be able to use the mcp2515 CAN controller with the TH8056 CAN transceiver. yeeaaahh, I think it will work.. just need to set mode0 and 1 to high on any digital 5v pin so its in normal 33.33 comms and the rest looks like I can follow the example schematic.

Might as well give that a go at the same time, looks like the TH8056 is standard size SOIC8.. time for board v2